What Is a Cold Sore?

Cold sores are caused by a viral infection named herpes simplex virus that attacks the skin and nervous system. Cold sores are small, painful, fluid-filled blisters usually on the mouth and last a week or 2. Although they're called cold sores, you don't need to have a cold to get one. Some people call them fever blisters, but you don't have to have a fever to have one, either. There are two types of herpes simplex infection: herpes simplex virus one (called HSV-1 for short) and herpes simplex virus two (called HSV-2 for short). Although both can cause cold sores around a person's mouth, most are caused by HSV-1